1. Field of the Invention
The present invention relates to a dual phase steel sheet and a method of manufacturing the same and, more particularly, to a technique for imparting dent resistance, low yield strength, high Ri value (Lankford value) and high formability to steel sheets for exterior and interior panels of automobiles.
2. Description of the Related Art
Since steel sheets for automobiles are generally subjected to pressing, the steel sheets are required to have excellent press formability, which is guaranteed by securing high ductility and high Ri value. In other words, the steel sheets for automobiles are high strength steel sheets and it is most important that they have both high ductility and high Ri value.
However, in high strength steel sheets satisfying requirements for weight reduction and passenger safety, the amount of alloy components such as Si and Mn increases and causes deterioration of formability and severe deterioration of plating characteristics, thereby making it difficult to produce steel sheets for automobiles that satisfy all of these requirements.
Since the steel sheets for automobiles are also required to have high corrosion resistance, hot-dip galvanized steel sheets exhibiting good corrosion resistance have been used in the industry. The hot-dip galvanized steel sheet is produced using continuous hot-dip galvanizing equipment, which performs recrystallization annealing and galvanizing on the same line, so that the hot-dip galvanized steel sheet has good corrosion resistance and can be processed at low cost. Further, a hot-dip galvannealing steel sheet produced through hot-dip galvanizing and reheating is also widely used due to its good weldability and formability in addition to good corrosion resistance.
As described above, to further reduce the weight of an automobile body while strengthening the body, there is strong demand for the development of high strength cold-rolled steel sheets having excellent formability and high strength hot-dip galvanized steel sheets having excellent corrosion resistance through a continuous hot-dip galvanizing line.
Recently, in the automotive industries, high strengthening of structural components and exterior panels for automobiles have been rapidly progressed in the course of attempting to achieve weight reduction and quality enhancement of automobiles. Here, there is demand for the provision of high strength steel sheets having good dent resistance to increase resistance to impact, which is caused by collision with an external object and results in damage of the exterior panel, by application of high strength steel to the exterior panel.
Further, since precise formation is important for an external appearance of an automobile, there is demand for the provision of bake hardening steel (BH steel) that has low strength to permit easy formation before painting and has increased strength after painting. Currently, the BH steel is developed to have a tensile strength of about 35˜450 MPa.
One example of high strength hot-dip galvanized steel sheets is a steel sheet that has a dual phase of soft ferrite and hard martensite and is produced by a method of manufacturing a hot-dip galvanized steel sheet having improved elongation (El) and Ri value (Lankford value). However, this technique cannot guarantee good galvanizing quality due to excess Si in the steel sheet and suffers from a problem of high manufacturing costs due to addition of a large amount of Ti and the like.